Diesel engine exhaust contains contaminants the emission of which is regulated for environmental and health reasons. Example contaminants present in diesel engine exhaust include particulate matter, nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO).
The particulate matter in diesel engine exhaust generally constitutes a soluble organic fraction (“SOF”) and a remaining portion of hard carbon. The soluble organic fraction may be partially or wholly removed through oxidation in an oxidation catalyst device such as a catalytic converter. However, catalytic converters are typically capable of producing a reduction of only about 20% of the total particulate emissions. Therefore, vehicles equipped with diesel engines may have exhaust aftertreatment systems that include diesel particulate filters for more completely removing the particulate matter from the exhaust stream, including the hard carbon portion. Conventional wall-flow type diesel particulate filters may have particulate removal efficiencies that exceed 85%.
In addition to particulate filters for removing particulate matter, diesel exhaust treatment systems can include structures for removing other undesirable emissions such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). Catalytic converters can be used to remove CO and HC. NOx can be removed by structures such as lean NOx catalysts, selective catalytic reduction (SCR) catalysts and lean NOx traps.
Lean NOx catalysts are catalysts capable of converting NOx to nitrogen and oxygen in an oxygen rich environment with the assistance of low levels of hydrocarbons. For diesel engine exhaust, hydrocarbon emissions are generally too low to provide adequate NOx conversion. Thus, hydrocarbons are typically required to be injected into the exhaust stream upstream of the lean NOx catalysts.
Lean NOx traps use materials such as barium oxide to absorb NOx during lean burn operating conditions. During fuel rich operations, the NOx is desorbed and converted to nitrogen and oxygen by catalysts within the lean NOx traps.
SCR systems typically use ammonia as a reductant for reducing NOx to N2. A typical SCR system includes an SCR substrate having an SCR catalyst such as an iron based catalyst, a copper based catalyst or other metal based catalyst. In a typical NOx reduction reaction, the catalyst provided on the SCR substrate promotes the reaction of ammonia with NOx to form N2 and H2O. This reaction is favored by the presence of excess oxygen. In certain SCR systems, a urea-water solution is provided as a reductant source for use in the reduction of NOx. However, to function as an effective reductant, the urea-water solution must be decomposed into a useable reductant form (i.e., ammonia). To decompose the urea-water solution, the water in the solution is first removed through vaporization. Next, the urea decomposes via thermolysis to form isocyanic acid (HNCO) and ammonia (NH3). Finally, the HNCO decomposes via hydrolysis to form NH3 and carbon dioxide. The effective decomposition of the urea-water solution to ammonia is assisted through effective mixing.
Regulations relating to diesel engine exhaust emissions are becoming increasingly more stringent. As the regulations on the level of contaminants in diesel engine exhaust become more stringent, it is often necessary to upgrade the diesel exhaust aftertreatment systems present on preexisting diesel engine powered vehicles. Retrofit exhaust aftertreatment systems are used to upgrade the level of emission control provided on existing diesel engine powered vehicles so as to comply with the ever changing emission control regulations.